G-protein–mediated inhibition of the Trp channel TRPM1 requires the Gβγ dimer

Abstract

ON bipolar cells are critical for the function of the ON pathway in the visual system. They express a metabotropic glutamate receptor (mGluR6) that, when activated, couples to the G o class of G protein. The channel that is primarily responsible for the synaptic response has been recently identified as the transient receptor potential cation channel subfamily M member 1 (TRPM1); TRPM1 is negatively coupled to the mGluR6/Go cascade such that activation of the cascade results in closure of the channel. Light indirectly opens TRPM1 by reducing transmitter release from presynaptic photoreceptors, resulting in a decrease in mGluR6 activation. Conversely, in the dark, binding of synaptic glutamate to mGluR6 inhibits TRPM1 current. Closure of TRPM1 by G-protein activation in the dark is a critical step in the process of ON bipolar cell signal transduction, but the precise pathway linking these two events is not understood. To address this question, we measured TRPM1 activity in retinal bipolar cells, in human ependymal melanocytes (HEMs) that endogenously express TRPM1, and in HEK293 cells transfected with TRPM1. Dialysis of the Gβγ subunit dimer, but not Gα o , closed TRPM1 channels in every cell type that we tested. In addition, activation of an endogenous G-protein-coupled receptor pathway in HEK293 cells that releases Gβγ without activating Go protein also closed TRPM1 channels. These results suggest a model in which the Gβγ dimer that is released as a result of the dissociation from Gα o upon activation of mGluR6 closes the TRPM1 channel, perhaps via a direct interaction. patch clamp | calcium imaging R etinal ON bipolar cells are connected to photoreceptors through a sign-inverting synapse. At this synapse, glutamate binds to the metabotropic glutamate receptor mGluR6, which couples to the closure of a cation-selective transduction channel. Insight into the molecular identity of the transduction channel was gained from the observation that low levels of mRNA encoding the transient receptor potential ion channel TRPM1 was associated with a form of congenital stationary night blindness (CSNB) in a population of Appaloosa horses (1). Soon after, it was shown that ON bipolar cell function was absent or severely impaired in a mouse model that lacked TRPM1 expression (2-4), although there is evidence that one or more classes of cone-driven ON bipolar cells may use an additional channel (3). Mutations in TRPM1 have now been positively linked to CSNB in humans as well (5-7). Taken together, studies of mouse, horse, and human provide convincing evidence that the ON bipolar cell transduction channel is composed of TRPM1, either alone or in conjunction with other channels. However, all these studies do not provide insight into the mechanism by which activation of mGluR6 leads to closure of TRPM1.The metabotropic receptor mGluR6 preferentially couples to the G o class of G protein, both in bipolar cells (8, 9) and expression systems (10, 11). Activation of mGluR6 and, correspondingly, Go, results in closure of TRPM1. However...

Depolarizing bipolar cell dysfunction due to a Trpm1 point mutation

“…7). The variability in response waveform is reflective of our experience across multiple studies (Rampino et al 2011;Shen et al 2009Shen et al , 2012 and may reflect variability in diffusion characteristics of individual puffs as well as the location of the puffer pipette with respect to the membrane patch being analyzed. In comparison, responses of Trpm1 tvrm27/tvrm27 DBCs under these conditions were indistinguishable from background noise (Fig.…”

The Expanding Roles of GβγSubunits in G Protein–Coupled Receptor Signaling and Drug Action

“…Although we will not discuss them in detail, a number of other canonical and noncanonical Gbg effectors have been identified, including GRK isoforms (Lodowski et al, 2003a,b), phosducin (Bauer et al, 1998), TRPM1 (transient receptor potential cation channel subfamily M member 1) channels (Shen et al, 2012), Na v 1.8 voltage-gated sodium channels (Belkouch et al, 2011), the WD40 repeat protein WDR26 (WD repeatcontaining protein 26) , Radil (Ahmed et al, 2010), ElmoE (Yan et al, 2012), A-type potassium channels , and glycine receptors (Yevenes et al, 2006). This list is likely to expand further.…”

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“…Thus, mGlu6 receptors localised postsynaptically on ON bipolar cells are thought to negatively regulate the opening of transient receptor potential cation channel subfamily M member 1 (TRPM1) [174], via the Ga o subunit [175], and/or via an interaction with the Gbc complex [176].…”